The present invention relates generally to a sole structure for a shoe, and more particularly, to an improvement in the sole structure for enhancing cushioning properties and bendability of the forefoot portion of the sole.
Japanese patent application laying-open publication No. 2003-339405 shows a sole structure for a shoe to secure cushioning properties and improve bendability. The sole structure shown in this publication has a structure in which an upper plate and a lower plate are disposed on the upper side and the lower side, respectively, of a wavy plate that extends from the heel region to the forefoot region.
In this case, a plurality of voids formed between the wavy plate and the upper and lower plate provide cushioning properties. Also, in this case, the wavy plate has a two-layered shank portion of a spindle shape in the sole midfoot portion. Such a shank portion restrains bending deformation of the sole midfoot portion, thus improving the bendability of the sole forefoot portion.
However, in the prior art structure, the sole forefoot portion also has a three-layered plate structure. During bending of the sole forefoot portion, the lower plate acts to restrict expansion and contraction of the wavy plate in the longitudinal direction. Therefore, it was difficult to fully enhance the bendability of the sole forefoot portion. Similarly, since the lower plate restricts deformation of the voids, it was also difficult to fully enhance the cushioning properties of the sole forefoot portion.
An object of the present invention is to provide a sole structure for a shoe that can improve bendability and cushioning properties of the sole forefoot portion.